It is sometimes necessary to change the designs of an air separating device during the design phase as a function of changing client requirements. For example, the client may decide that it needs to produce argon, even if the device has already been designed without providing for argon production or without an argon column.
One way of addressing this problem is to design the device with a column that is able to produce argon, but that discharges the argon-rich gas into the waste nitrogen if the argon is not required. It is also possible to increase the dimensions of the low-pressure column. These two solutions necessarily increase investment costs.
An air-distillation facility enabling argon production usually comprises a medium-pressure column, typically operating under approximately 6 bars absolute, surmounted by a low-pressure column typically operating slightly above atmospheric pressure and coupled to an impure-argon production column. An evaporator-condenser creates a heat-exchange relationship between the overhead vapor of the medium-pressure column, comprising substantially pure nitrogen, and the kettle liquid from the low-pressure column, comprising substantially pure oxygen.
The low-pressure column includes a distillation segment and, immediately above same, a first intermediate distillation segment, and several segments above the first intermediate distillation segment, usually two or three.
Each of the distillation segments is made of cross-corrugated structured packing blocks.
As is well known, a cross-corrugated packing block is made of a packet of corrugated plates each arranged in a substantially vertical plane and touching one another, each plate having a substantially rectangular shape. The plates are corrugated obliquely, and the direction of incline of the corrugations is inverted from one plate to the next. All of the plates are of the same height, while the length or horizontal dimension of same increases from a minimum value for an end plate to a maximum value for the middle plate, before decreasing to the same minimum value for the other end plate.
Each of the segments is a continuous packing segment, i.e. a segment comprising a direct stack of elementary blocks on one another with no intermediate fluid redistribution device, each elementary block being turned 90° about the axis of the column in relation to the two adjacent layers.
The first intermediate distillation segment, as described in EP-A-0664144, has a smaller section than the other segments and is therefore in the middle of the low-pressure column with an annular-section space between the edge of the segment and the main shell of the column. The column is designed such that the argon-rich vapor can be drawn off in this annular-section space beneath a baffle that divides the space into a lower section and a vertically upper section. This vapor is then fed into the argon column. The kettle liquid in the argon column is also returned to the lower section, where the most argon-rich vapor is drawn off. The vaporized rich liquid coming from the top condenser of the argon column is conveyed to the upper section of the space.
The first intermediate distillation segment is separated from adjacent segments by distributors.
Consequently, the gas extractions and feeds in the column that are related to argon production do not increase the height of the column.
The reduced diameter of this first intermediate distillation segment is possible without increasing the diameter of the column because this segment is not design critical. Indeed, a significant quantity of gas flows into the argon column and therefore does not pass through this segment.
If the facility is designed not to produce argon, this reduced-section segment is not installed, according to the prior art.